1. Field of the Invention
The present invention relates to a communication control method and a communication control apparatus therefor in which a master station and slave stations are connected to one another through a communication channel having a star configuration so that each slave station accesses the master station through the communication channel of TDMA (Time Division Multiple Access) system, and the master station can transmit data to all the slave stations through the communication channel in a broadcast mode.
2. Description of the Prior Art
FDMA (Frequency Division Multiple Access) system and TDMA system are respectively known as a communication control system for assigning one channel to plural stations. Among the TDMA system which has been used as a conventional communication control system for time-dividing one channel of a satellite in a satellite communication, an assignment of time slots to each station is performed according to, for example, a fixed assignment method by which time slots are assigned in a fixed manner (Per Assignment) or a connection request assigning method by which time slots are assigned in the arrival order of a time slot assignment request (Demand Assignment).
In the conventional communication system, the assignments of the time slots to the stations are performed as explained above. Therefore, even when the emergency level of data to be transmitted is higher, the time-slot assignment is performed in the same manner as data whose emergency level is lower. Accordingly, even data having a high emergency level cannot be transmitted immediately, or rather, until its transmission turn comes. Therefore, a standby time needed until the data having the high emergency level are transmitted becomes longer, and a delay time for the data transmission becomes also longer.
An object of the present invention is to provide a communication control method and a communication control system apparatus therefor which can reduce a time-slot assignment waiting time of a time-slot assignment request when a highly-emergent communication is made, and can flexibly vary the occupation rate of time slots having high emergency level in accordance with the condition of the time-slot assignment request to enhance the usage efficiency of the time slots.
According to a first aspect of the present invention, there is provided a communication control method in which plural slave stations transmit data to a master station through a transmission channel of a time division multiple access system, the master station transmits data to the slave stations in a broadcast mode, the master station assigns a time slot to a time slot assignment request transmitted from each of the slave stations to the master station, and each of the slave stations performs the communication by using the time slot assigned thereto, which method comprises: an initial priority assigning step for beforehand assigning initial priorities to time slots, the initial priority assigning step being executed by the master station; a priority assigning step for assigning a priority to a time slot assignment request to be transmitted from each of the slave stations to the master station in accordance with the emergency level of a communication, the priority assigning step being executed by each of the slave stations; a priority judging step for judging the priority assigned to the time slot assignment request when the master station receives the time slot assignment request to which the priority is assigned in the priority assigning step, the priority judging step being executed by the master station; and a time slot assigning step for assigning a time slot having the same priority as the judged priority to the time slot assignment request when the time slot having the same priority is not occupied, whereas assigning a time slot having a priority which is lower than the judged priority to the time slot assignment request when the time slot having the same priority is occupied, but at the same time the time slot having the priority which is lower than the judged priority is not occupied, the time slot assigning step being executed by the master station.
According to the communication control method of the present invention, the time slot having the priority which is lower than the priority which is set in the time slot assigning request is allocated to the time slot assigning request so as to increase the capacity of a communication whose emergency level is high, and shorten the time slot assignment waiting time of the time slot assignment request having a high emergency level.
According to a second aspect of the present invention, there is provided a communication control apparatus in which plural slave stations transmit data to a master station through a transmission channel of a time division multiple access system, the master station transmits data to the slave stations in a broadcast mode, the master station assigns a time slot to a time slot assignment request transmitted from each of the slave stations to the master station, and each of the slave stations performs the communication by using the time slot assigned thereto, wherein the master station comprises: a judgment unit for judging a priority extracted from a time slot assignment request whose priority is set in accordance with an emergency level of a communication and which is transmitted from each of the slave stations; and a slot assignment controller for assigning an initial priority to the time slot beforehand, and assigning a time slot having the same priority as the judged priority to the time slot assignment request when there is a time slot having the same priority as the judged priority, whereas assigning a time slot having priority which is lower than the judged priority to the time slot assignment request when there is not any time slot having the judged priority and at the same time there is a time slot having priority which is lower than the judged priority.
According to the communication control system apparatus of the present invention, when any time slot whose priority is the same as the priority which is set in the time slot assignment request is not vacant, but at the same time a time slot whose priority is lower than the priority concerned is vacant, the time slot whose priority is lower than the priority concerned is assigned to the time slot assignment request, thereby shortening the time slot assignment waiting time of the time slot assignment request which is high in emergency level, that is, in priority, and also enhancing the usage efficiency of the time slot.